In a mobile communication system, a base station transmits and receives signals of multiple users at a fixed frequency fc, and a user equipment tracks the frequency of a received signal through Automatic Frequency Control (AFC). In a high-speed movement context, the base station transmits a signal at the frequency fc, and the user equipment receives the signal at a frequency of fc+Δf over a channel, where Δf is a Doppler frequency shift, and both the transmission and reception frequencies of the user equipment settle down around the frequency fc+Δf of the received signal after a period of time elapses. Thus the user equipment transmits a signal at the frequency of fc+Δf, and the base station receives the signal at a frequency of fc+2Δf over an uplink channel, but the reception frequency of the base station is fc, that is, the base station receives the signal with the maximum frequency deviation of 2Δf, as illustrated in FIG. 1.
A user equipment moves away from a base station prior to a cell handover, and the reception frequency of the user equipment settles down around fc−fd as calculated from a Doppler frequency shift. The user equipment moves to a new base station after the handover and receives a signal with a frequency of fc+fd, but the user equipment receives the signal still at the frequency of fc−fd due to the delay of the AFC, thus the user equipment detects the signal with a frequency deviation instantly increased to the maximum value of 2fd, as illustrated in FIG. 2, where bracketed values represent the transmission and reception frequencies of the base station or the user equipment.
The jump of the frequency deviation influences the user equipment due to the sudden increase of the frequency deviation, which causes the performance of the user equipment to deteriorate sharply in a period of time after the handover so that a user may suffer from a degraded experience and even a dropout. Taking a Time Division-Synchronization Code Division Multiple Access (TD-SCDMA) system as an example, the frequency deviation may be up to 1500 Hz in high-speed movement context of 400 Km/h, and the demodulation performance of the user equipment may deteriorate seriously, therefore a solution of frequency deviation pre-calibration at the base station has to be introduced to improve the demodulation performance of the user equipment.
As illustrated in FIG. 3, a fundamental idea of frequency deviation pre-calibration lies in that the base station estimates the Doppler frequency deviation of fd over an uplink channel of a target user and pre-calibrates the frequency of a signal for downlink transmission to the user by the estimated frequency deviation, that is, the downlink transmission frequency is adjusted to fc−fd to pre-compensate for the influence of the downlink frequency deviation so that the user equipment receives the signal with a frequency around the frequency fc and there is no significant influence of the Doppler frequency deviation upon the signal detected by the user equipment to thereby improve the detection performance of the user equipment.
With frequency deviation pre-calibration at the base station, the user equipment hardly experiences an obvious change in the frequency deviation during the cell handover, and here FIG. 4 illustrates a schematic diagram of frequency deviations of transmitted and received signals of the base station and the user equipment after and before the handover, where bracketed values represent the transmission and reception frequencies of the base station or the user equipment. The user equipment transmits a signal at the frequency fc, and the base station receives at the frequency fc the signal with the frequency of fc+fd over an uplink channel and can estimate a frequency deviation of fd. The base station transmitting data to be transmitted to the user in the downlink adjusts the transmission frequency to fc−fd so that the user equipment receives the transmitted signal still at the frequency fc over a downlink channel. The user equipment operates at a frequency stabilized around the cell frequency fc prior to the handover and at a new cell frequency after the handover to thereby alleviate the change in the frequency deviation of the user equipment and to improve the demodulation performance of the user equipment. This solution improves the performance of a network at the cost of increased complexity of the base station.
FIG. 5 illustrates a schematic diagram of a change in a Doppler frequency shift over a high-speed movement channel, and taking a TD-SCDMA system as an example, a frequency locking process is performed for a powered-on user equipment over a broadcast channel, e.g., a Downlink Pilot Time Slot (DwPTS), a Primary Common Control Physical Channel (PCCPCH), etc., and a local oscillator of the frequency locked user equipment operates at the frequency of a received signal. As can be apparent from the relationship of the change in a Doppler frequency shift in FIG. 5, the user equipment is locked at a frequency changing with the location where it is powered on, and the frequency ranges from fc−fd to fc+fd in Hz, where fc represents the frequency of a signal transmitted from the base station and fd represents the maximum Doppler frequency shift.
The existing solution of frequency deviation pre-calibration can not be applied to the broadcast channel, and therefore with the Doppler frequency shift fd of the channel, the user equipment is locked at the frequency of fc+fd, and here a frequency deviation of 2fd is estimated at the side of the base station. If a service is being set up while performing a frequency deviation pre-calibration process over a service channel, then transmission over the service channel is at a frequency of fc−2fd, and the user equipment receives a signal at the frequency of fc−fd over the channel, but the local oscillator of the user equipment operates at the frequency of fc+fd. Here the demodulated signal of the user equipment may be subject to a frequency deviation of 2fd, which is approximately 1500 Hz at a vehicle speed of 400 Km/h in the TD-SCDMA system, thus the performance of detecting the signal at the user equipment may deteriorate seriously.